1. Field of the Invention
The present invention relates to a method and an apparatus for forming an image by electrophotography, wherein the temperature of an image fixing heating device is controllable in a temperature-control-on mode and non-controllable in a temperature-control-off mode. More particularly, the invention is concerned with such electrophotographic image forming method and apparatus which permit a new image forming job to be started in a relatively short time after the last image forming job which is requested while the apparatus is in the temperature-control-off mode.
2. Discussion of the Related Art
A known apparatus such as a laser printer having an image forming device adapted to form a visible image by electrophotography is generally provided with a heating device for applying heat to a recording medium for fixing the visible image on the recording medium. The temperature adjacent to or within the heating device is controlled to be held at a first level (stand-by temperature) when the apparatus is in a stand-by mode without a print start command being received, and at a second level (image fixing temperature) higher than the first level when the apparatus is in a printing mode with an image forming operation being performed.
For instance, the heating device has a heat generator in the form of a halogen lamp, which is turned on and off to control the temperature adjacent to the heating device. The amount of consumption of electric power required for the heating device is as large as 70-80% of the total amount of electric power required for the apparatus. This means a considerable waste of energy while the apparatus is in the stand-by mode in which the heating device is controlled to hold its temperature at the first level indicated above. To minimize this waste, it has been proposed to provide means for providing a temperature-control-off mode if the apparatus is held in the stand-by mode for more than a predetermined time, namely, if print data (with a print start command) have not been received from an external device for more than the predetermined time, or alternatively if an appropriate operator-controlled switch is operated. The heating device is controlled to control the heating temperature in the temperature-control-on mode, and is not controlled in the temperature-control-off mode.
In the apparatus having the temperature-control-on mode and temperature-control-off mode as described above, the amount of waste of the electric power is reduced since the heating device is held off after a predetermined time has passed without a print start command being received or after the appropriate switch is operated.
When the above apparatus receives a print start command with print data from an external device, the temperature adjacent to the heating device is first controlled to be held at the predetermined first level (stand-by temperature), and then a scanner motor for sweeping an image beam on the basis of the print data is started. The temperature is then controlled to be held at the second level (predetermined image fixing temperature).
If the print start command and print data are received from the external device after the temperature-control-off mode is established, the heating device is first turned on to raise the temperature to the first level, and the scanner motor is then turned on. The temperature is then further controlled to be raised to the second level or nominal image fixing temperature. In other words, the scanner motor is not turned on until the temperature has reached the first level, that is, the scanner motor is not turned on as soon as the print start command is received. Accordingly, it takes a relatively long time for the temperature of the heating device to be raised to the image fixing level and for the image forming operation to be started (on the first sheet of paper, for example) after the print command is received in the temperature-control-off mode.